1. Field of the Invention
The invention relates to a method for the partial fusion of objects.
2. Description of the Prior Art
In known such methods a substantially continuously flowing plasma is used, mostly for hardening the surface of objects made of steel.
A laser beam or an electron beam is mostly used for other methods, e.g. for welding, in particular for spot welding thin sheets, or for producing a breakthrough in thinner metallic objects. This leads to the disadvantage, however, that laser welding processes require a very laborious preparation of the parts to be welded, which must be joined with a very high precision in order to enable their welding by means of a laser beam. The same also applies with respect to methods using electron beams. Moreover, the equipment required for performing such methods is very complex in a constructional respect.
It is the object of the present invention to avoid such disadvantages and to provide a method of the aforementioned kind which allows a simple machining of objects, in particular the production of spot welds or the burning of breakthroughs.
This is achieved in accordance with the invention by machining with merely on plasma pulse, which is produced by applying a voltage pulse exceeding the arc-over voltage of the gap between the electrodes.
As a result of the proposed measures it is possible with relatively roughly prepared parts to join the same by means of spot welding. Measures will substantially suffice as are also required in electric resistance spot welding.
A very high ejection speed of the plasma pulse is secured by the ignition of the arc by exceeding the arc-over voltage of the anode-to-cathode gap, so that this pulse will impinge upon the parts to be welded with a high kinetic energy. The plasma pulses thus produced will reach very high temperatures of 20,000 to 50,000xc2x0 C. and will cause adequate fusion of the mutually adhering surface areas of the parts to be joined despite a short action period of e.g. 10xe2x88x925 to 10xe2x88x920 seconds and will thus cause a secure connection.
Machining in a protective gas atmosphere helps avoid the formation of oxide layers on the subjects, with the gas used for the production of the plasma, mostly argon or helium, appropriately being simultaneously used as inert gas.
If the plasma pulse has a duration of about 10xe2x88x925 to 10xe2x88x920 seconds, preferably 10xe2x88x924 to 10xe2x88x921 seconds, relatively compact devices or plasma torches may be used which can be operated at a relatively high output over a short period.
For a weld seam from a number of welding spots, the object to be joined are charged with a number os successive plasma pulses while the objects are moved relative to the electrodes and the electrodes are kept at a constant distance from the objects, a repetition frequency of the plasma pulses of 5 to 100 Hz being provided.
In such a device it is possible in a simple way to charge the subject(s) to be machined with a sequence of very short plasma pulses. In the course of charging the capacitor battery the arc-over voltage of the anode-to-cathode gap will be exceeded and thus an arc will be formed through which there will be a discharge of the capacitor battery. The arc will extinguish as soon as the voltage of the capacitor battery drops below the arc drop voltage. As a result of a respective dimensioning of the charging circuit and the discharge circuit of the capacitor battery with respect to the time constants it is possible to determine both the arc duration in each cycle as well as the repetition frequency. The arc which thus burns only very briefly produces plasma pulses which, as a result of the very rapid heating of the ambient gas, exit with a very high speed from the outlet opening of the chamber of the plasma torch and impinge upon the objects to be joined or the object to be provided with a breakthrough and as a result of their high temperatures ensure the fusion or the melt-through of the object(s).
Short pulse durations of the plasma pulses of 10xe2x88x925 to 10xe2x88x920 seconds for example and a repetition frequency of 7 to 100 Hz are required for the careful treatment of the objects to be machined. As a result of these short operating times of the individual plasma pulses, the thermal stress on the objects is kept low and thus the danger of distorting the mostly very thin or thin-walled objects is substantially avoided.
Initiating even before reaching the arc-over voltage of the anode-to-cathode gap allows keeping the arc duration, and thus the plasma pulses, extremely short without having to make any particularly great efforts concerning a particularly low-resistance arrangement of the discharge circuit of the capacitor battery.
It is principally also possible to also use a technical AC network or a voltage source supplying a high-frequency AC current in conjunction with a phase controller instead of the capacitor battery as a voltage supply for the plasma torch. In this respect it must be ensured in the case of electrodes made of different materials that merely equally polarised half-waves are partly connected through so that voltage pulses with the same polarity are always applied to the different electrodes and substantially the same ratios as in the supply of the plasma torch with DC voltage pulses, like from a capacitor battery for example, are obtained.
In cases in that both are electrodes made from the same material, pulses with different polarity can be applied to each of the two electrodes.
As electrodes which are made of different materials for the purpose of achieving a longer service life are usually charged with the same polarity in plasma torches, the terms xe2x80x9canodexe2x80x9d and xe2x80x9ccathodexe2x80x9d are generally used in the description and the claims.